Engine cuts out for 1/10th of a second

[Patronus]

By this time everybody knows my set-up. '85 Carrera, nice dependable condition. Engine is smooth, car is strong, nothing to complain about.

Yesterday, at normal operating temperature, just coasting casually at 100mph. Sudden, complete and unexpected engine cut-out for about 1/10th of a second, but then everything's fine again. 15 minutes later, same thing at a different speed. In total it happened about 5 times yesterday, at different speeds & different conditions.

Are these the typical symptoms of a head temp sensor going bad? I had it replaced 6 months ago.

[jluetjen]

I used to have that problem in my Fiesta and it was the ignition box going bad. The problem only went away after replacing that. I doubt that it's plugs or the plug wires since my experience is that those pieces usually cause rough running rather then no running.

What sort of rev limiter is in the Carrera -- spark or fuel?

If that doesn't work, based on some other threads posted recently, you could always try replacing the airbox straps...

[JackT]

Did it cut out under load? If so, it could possibly be the plug wires, however, it would be a consistant miss if it is the wires.

[IROC]

Is this a symptom of a DME relay on the verge of crapping out? Or does the DME relay show up as a start/no start kinda thing?

Mike
'76 Euro 911 w/3.2

[BER]

I had this same problem on my '84 Carrera a few years back. Engine would cut out for a fraction of a second. I noted it happened wheneven I hit small bumps or uneven seams in the pavement.

The problem was a loose wire at the fuse block in the front trunk. I found the problem by idling the car and jiggling all the wires at the fuse block (a couple at a time) until I found the problem.

Before finding the actual problem, I went through all the usual 3.2 ltr suspects...DME relay, all ground connections, head temp sensor, O2 sensor, plug wires, etc.

Good Luck, Bruce

[HawgRyder]

When an engine cuts out that quickly and then resumes...it has to be electrical.
If it was fuel related...the interval would be slower.
Bob

[ted]

Already check cap and rotor?

[gr8fl4porsche]

I agree with the suggestion to replace the airbox straps

Just cured (hopefully) this same problem on my '87.

Although I worked on several items at once I believe it was the plug wires. I also changed all the usual suspects, grounds, dme relay, cap, rotor, plugs, checked for loose wires, cleaned all electrical contacts I could find, temp sensor was changed right before I bought the car.

Everytime I worked on the car it seemed to fix the problem only to have it resurface again. For almost a year I was nervous to downshift and stand on it because when it cut out, the seatbelt was the only thing keeping me from going through the windscreen, at least thats what it felt like.

New Magnecores and all seems to be well. Took it out a couple of weeks ago for several 0-100-0 runs and it ran beautifully.

[SpeedracerIndy]

My fathers Carrera had this very same problem. I turned out to be moisture in the distributor cap. I have heard of Carreras not running after car washes or bad rains because of this also. I opened up the distributor cap and let is sit open over night and the problem hasn't come back since. I also just resently put a smear of silicone greese (not sealant!) around the cap so that it will seal against the distributor body. Hopefully it is something as simple as that and you do not have to start chasing down electrical gremlins!

ps. I thought air box straps were last used on the SCs.

[88911coupe]

Same symptoms on my 88 some time ago. Turned out to be an extremely fine crack in one of the solders in the DME unit itself (not the DME relay) I'd still change the relay since it's cheap and easy. I did that on my car but it was no help. Several on this board walked me through finding the crack and you can probably do a search. If not then PM me and I can give you more info.
Later

[Stu Wilkinson]

Try checking your earths for tightness and cleanliness.
I have had similar problems in the past and these checks worked for me.

[Patronus]

Thanks for all the advice, I'll be reading through it very carefully. I just returned from my trip, and I have lots of info about my problem. Perhaps you can make sense out of it.

On my way to my destination (Friday) the car was fine for about 10 minutes, and then the problem started again. Every 10 minutes or so it would cut out, just as I described earlier. After about 1.5 hours of driving it became very bad, with the car cutting out a few times per minute. But it never died. Fortunately I arrived at my destination soon after it became bad.

I noticed that, every time it happens, the rev counter would fall to almost zero. Insantly. And then recover just as quickly. The warning lights don't come on in the process, so I assume the engine doesn't die.

On Saturday we drove out about 40 miles (highway) and the car started cutting out after a few minutes, and it became quite bad - a few times per minute. Then a strange thing happened - as I pulled into the driveway at my destination, the revs went up and down madly, between 1000rpm - 3000rpm. I had to work real hard not to drive into other parked cars, since all of this happened in 1st gear.

Later that day we drove 40 miles back, and the car behaved badly again. At some stage I tried to overtake slower traffic and my car had no power, sitting on 4000rpm no matter how hard I push the pedal. This was not a 10th of a second problem any more, it lasted about 5 seconds. But, directly after that, the problems went away and the last 20 miles were incident free.

20 Minutes later, when I needed to drive to he shop, the car would't run properly so I turned back.

This afternoon I had to drive back home, about 150 miles. The first 5 minutes were fine, but then it became REALLY bad. The revs would fall to about 200rpm (not 2000) and somehow stay there for a few seconds before the car would die. I had to switch the car off and re-start it, on the fly. It wouldn't re-start by itself. It fired up every time I turned the key, however. But it happened about every two minutes.
The first 40 miles were hell, and then suddenly the problem went away. The remaining 110 miles were a real pleasure. Not once did the car cut out.
It had now been standing of 30 minutes, and I drove it around the block. Again it cut out about three times.

Info to work on:

1. At first it only happened at constant speed, at operating temperature. Now it happens at any temperature (never within the first 2 minutes of driving, however) at any throttle position, including zero throttle and full throttle.

2. When it happens the rev counter instantly falls to zero and then recovers immediately thereafter. All in less than a second. Except for the times it lingered around 200rpm and then died.

3. Sometimes the revs would jump between 1000rpm and 3000rpm. Not oscillate - JUMP.

4. The warning lights never come on, so the car doesn't really die during these cut-outs. Only when the car actually died (see 2) did the lights come on.

5. There's no smoke, sound, smell or anything else out of he ordinary. Not even during a cut-out.

6. I checked all DME connections, and cleaned them.

7. The road surface has no effect on the problem. Neither has g-force, speed, rpm, outside temperature or (apparently) engine temperature.

Perhaps you can help me in the light of the above info. I will now go through the advice I received, for which I thank you all again.

[SpeedracerIndy]

Sounds a little more serious than moisture in the distributor cap. It does sound electrical though. I assume you have already checked the cap for cracks, rotor, wires, and plugs. Sorry I can't offer any more assistance, but someone here will be able to. Good luck.

[RoninLB]

I know very little about the DME.. but I had the exact same problem and symptoms.. changed CD box and coil at the same time/ done deal.............Ron

[350HP930]

If you are seeing a drop in the tachometer at the same time then I would certainly think that the problem lies in the ignition system.

Begin your search there.

[Patronus]

I also think the tach dropping each time the car cuts out says something about the ignition system. I just checked the car running in the dark (a good start I guess) and there's no arcing or sparks. I replaced the plug wires recently and the condensors were fine.
The rotor is fine and so is the distributor cap. Just checked them both.

Can a defective coil do this type of thing? I mean, it's the next thing in the ignition chain.

[Quicksilver]

The coild wouldn't cause the tach to drop. The one kind of cheap thing I can think of is the crank sensors at the flywheel. If they are marginal they might cause something like this. I would check their connectors too. (To the left of the heater blower motor in the engine compartment).

Two other things would be to check the grounding straps at the rear of the left intake plenum and the strap at the front of the transmission.

I did have a weird cut out that sounded similar to this one except it didn't effect the tach. The fuel injection harness connector above and in front of the engine was loose enough to bounce around. I put a tye wrap around it. I don't think that could be it though.

Good luck
Wayne

[Patronus]

I spoke to a few people (my mechanic among others) and they reckon a faulty DME relay might cause problems like these. I'm going to put in a new one, as they are not too expensive, and see if that helps. Unless I notice that the current one is not properly in place or something obvious like that.

[Patronus]

88911 you were almost right. The problem turned out to be loose contact points (solder spots) on the DME relay. I soldered the two small relays back into position and my problem is solved.